NEURONAL METABOLISM 



.817 



'brain metabolism' is in essence largely 'dendritic 

 metabolism' (79). 



In the case of hypothalamic tissue, the oxidative 

 and glycolytic metabolism of the supraoptic and 

 paraventricular nuclei does not differ greatly from 

 that of the surrounding tissue (108, 110). Certain 

 fiber tracts such as the nonmyelinated tract in 

 Amnion's horn and the optic tract are especially rich 

 in lactic and malic dehydrogenase, as are the den- 

 drites, while, in contrast, glutamic dehydrogenase is 

 highest in myelinated nerves and the molecular 

 layer of Amnion's horn (110). Aldolase activity 

 parallels malic and lactic dehydrogenase in the 

 Amnion's horn as well as in myelinated fibers and the 

 optic tract. 



Among other enzymes which are present in ap- 

 preciable quantities in white matter are adenosine- 

 triphosphatase (6, 7, 88, 100) and particularly 

 purine nucleoside phosphorylase (iOl). Studies on 

 the cytoarchitectonic pattern of enzymes in the rat 

 cerebral cortex have served in part to correlate 

 particular enzyme systems with neural cell types. 

 The distribution of many respiratory enzymes and 

 phosphatases appears to be related to the protein 

 content in the various layers of the cerebral cortex 

 and to be a mirror image of the lipid concentration 

 (98, 100). The distribution of dipeptidase correlates 

 with the pattern density of neuronal and neuroglial 

 cell bodies, while acetylcholinesterase parallels the 

 distribution of the axons and dendrites, including 

 their ramifications (16). Acetylcholinesterase seems 

 to be associated to a considerable extent with motor 

 end plates and perhaps other synapses (16, 71); 

 and, although its appearance embryologically is 

 correlated with the development of neurons (45), it 

 is also present in large amounts in white matter 

 (54, 57). A porphyrin (probably coproporphyria 

 has been found exclusively in the white matter, and 

 largely in oligodendroglia (69). The reason for the 

 existence of porphyrins in the brain is not at all clear, 

 but they are believed to be involved in the photo- 

 stimulation of mammalian and avian sexual cycles, 

 so that the central nervous system itself may actually 

 be a 'tissue of perception' (35, 91). 



The retina is particularly useful for studying the 

 quantitative histochemistry of neural tissue since the 

 various components of the neuron are discreetly 

 separated in this structure (80). Malic dehydrogenase 

 and transaminase (80, 120), as well as succinoxidase, 

 are especially concentrated in the layer consisting of 

 the inner segments of rods, an area which is extremely 

 dense in mitochondria (107). Lactic dehydrogenase 



and phosphoglucoisomerase, which appear to have a 

 reciprocal relationship to malic dehydrogenase and 

 which are indicative of overall glycolytic metabolism, 

 are especially concentrated in all the inner layers of 

 the rabbit retina (80). The outer segments of the rods 

 and cones are deficient in all of the enzymes men- 

 tioned above, suggesting that, perhaps, these struc- 

 tures are metabolically dependent upon adjacent 

 layers (80). In general, the activity of isomerase and 

 malic and lactic dehydrogenase isomerase was 

 several times greater in retina than that in whole 

 brain, whereas glutamic-aspartic transaminase was 

 considerably lower. 



MITOCHONDRIA 



As the neuroblast is transformed into the adult 

 neuron and the nervous system begins to exhibit 

 functional characteristics, a number of dramatic 

 changes in its enzymatic pattern take place. With 

 the appearance of the dehydrogenases, cytochrome 

 oxidase, adenosinetriphosphatase and other mito- 

 chondrial enzymes, the main pathway of metabolic 

 energy shifts from anaerobiosis to aerobic glycolysis, 

 thereby tremendously increasing the 'efficiency' of 

 energy production. In the early embryonic develop- 

 ment of the neuron most of the nuclei have presumably 

 reached maturation, as indicated by its high DNA 

 content; and with the development of cytoplasm and 

 dendrites one would expect a corresponding increase 

 in the mitochondrial population. 



With the demonstration that oxidative phos- 

 phorylation of all tissues, including brain (4, 18) and 

 peripheral nerve (6), is carried on by the mito- 

 chondria, the problem of energy production within 

 the neuron and its localization has been placed in a 

 new perspective. The concept that mitochondria are 

 the sites of energy production within the nerve 

 originated almost a half century ago (87), and 

 neuroanatomists since that time have been concerned 

 with their intraneuronal distribution. [See Abood & 

 Gerard (6) for review.] Mitochondria are most 

 abundant in the nerve cell body and dendrites, and 

 although present throughout the axoplasm and 

 neurilemmal sheath, they are concentrated in certain 

 functionally significant areas such as the Schwann 

 cell (87), nodes of Ranvier (37), at the terminal 

 boutons (13), and the general area of the myoneural 

 junction (13). Quantitative data on mitochondrial 

 distribution in neural tissue is lacking, although a 

 correlation has been found between mitochondrial 



